1. Field of the Invention
The apparatus process of the present invention relates to filtration. More particularly, the process and apparatus of the present invention relates to an apparatus and process for filtering completion and workover fluids utilized during oil and gas well drilling and completion operations through a filtration system having a novel filtering element and system including filtering and cleaning capabilities therein.
2. General Background
Completion and Workover Fluids such as Calcium Chloride (CaCl.sub.2), Calcium Bromide (CaBr.sub.2), and Zinc Bromide (ZnBr.sub.2) are utilized during oil and gas well drilling, completion and well workover operations. The virgin completion fluid becomes contaminated with suspended particulates as it is injected into the well bores and subsequently recirculated. The industry has determined that penetration of certain sized suspended solids contained in the fluid during the injection and recirculation procedures may ultimately impair the productivity of the well. When the matrix of the formation is clogged with suspended particles, the formation becomes less permeable and yields are typically lower. In order to avoid this problem, filtration of completion fluids is an accepted practice in the industry.
Basically the industry employs two filtration techniques to remove suspended matter from completion fluids. These are "Multiple Pass" Filtration and "Single Pass" Filtration.
In Multiple Pass Filtration, suspended solids are filtered out by passing the fluid through a series of filter elements having varying particle retention capabilities. The idea is to first remove the larger particles followed then by decreasing retentions until such time that the desired final particle capture is achieved. A typical system would consist of a 50 micron element, followed by a 25 micron, then a 10 micron and finally a 2 micron. The filter elements are fitted into a steel tank commonly referred to a "Pod". Generally the system will have two pods so that one unit can remain in service filtering while the neighboring pod is out of service during filter element replacement. The filter elements are made of various materials including plastic, inorganic, and natural fiber microfilament. The particle retention rating is either by "Nominal" or "Absolute."
The National Fluid Power Association, (NFPA) defines "Absolute Rating," to be determined by the largest, hard, sperical particle that passes through the filter. Nominal retention rating is defined as an arbitrary micrometer value. In essence, this means that a nominally rated filter element may very likely be incapable of retaining particles for which its rating signifies, because nominal rated filters have the common deficiency of passing particles significantly larger in size, let alone particles that it represents to retain. Absolute rated elements will retain particles for which its rating represents provided of course that quality assurance is maintained during the mass production of such filters.
However, due to the expense involved, the use of the absolute element is typically confined to the final step in a multiple pass filtration system. The coarse filtration steps would usually be done with the nominally rated elements.
The replacement type cartridge filter elements, nominal as well as absolute, have by virtue of their physical size and configuration, a rather limited solids loading capability. Therefore, extensive use of cartridge type filters is usually confined to low solids filtration applications.
In those instances where cartridge filters are employed on high solids applications, the resulting short filtration cycles mean frequent element replacement, extensive manual labor and loss of production. In the drilling industry it is very common to have large amounts of suspended solids in the completion fluids, in some instances, as much as several thousand parts per million. A cartridge filter system utilized under these conditions would be very expensive to operate in terms of replacement element cost, labor involved with element changing and time lost during the filter turnarounds. In many instances, added rig time costing as much as $50,000 a day and more is directly attributed to inadequate filtration practices.
Despite the many short comings of the present filter methods, the drilling industry continued to use cartridge filters in the multiple pass systems.
Single Pass Filtration techniques were introduced into the drilling and completions industry in the early 1980's. The industry considered this new method to be a major improvement over multiple pass cartridge filter practices. See SPE 10648 "New Concept-High Density Brine Filtration Utilizing A Diatomaceous Earth Filtration System," Copyright 1982, Society of Petroleum Engineers of AIME.
A single pass concept which employs the use of filter aid material such as Diatomaceous Earth, (D.E.) or Perlite in filter presses and pressure leaf filters are commonly referred to as "D.E. Filters." The term "Single Pass Filtration" denotes the removal of undesirable suspended particulates in a completion fluid by passing the fluid through the filter one time rather than in cascading fashion like the aforementioned multiple pass cartridge filter method. A typical single pass filter is described in U.S. Pat. No. 4,428,425, which would generally comprise a filter press unit consisting of a structural steel framework suited to support a multitude of filter plates. Prior to starting the filtration cycle, the filter plates are compressed together and held securely in place by hydraulic force. The major portion of each filter plate is recessed. As the plates are compressed, the recessed areas form cavities or cake chambers. These cake chambers have a relatively large solids holding capacity. The filter press is therefore conducive to depth filtration and thus considerably much longer filter cycle lengths than cartridge type element filters. The surfaces of the filter plates are covered with a synthetic fabric such as Polypropylene. The function of the filter cloth is to retain the filter aid precoat material which becomes the actual filtering medium once the precoat has been uniformly deposited on the cloth surfaces.
One short coming of this apparatus is that as solids are retained and accumulate on the precoated cloths a rapid pressure differential across the filter will occur and the filtrate flow will be reduced greatly unless permeability of the cake being formed within the chambers is maintained. It should be noted that in those instances where extreme high solids are experienced, the on stream filtration time can be very short, even with proper body aid dosages. The filter press would then have to be pulled off line for cleaning and turnaround just about as often at it is in service filtering. Due to cost considerations and rig space limitations, it is generally impractical to employ standby presses in order to maintain filtration production while the press is in the cleaning and turnaround modes.
In cleaning, air or natural gas passes through the sealed cake chambers driving fluid contained within the press forward. The hydraulic clamping pressure is then released and the head plate is retracted creating an open space inbetween the filter chambers. The plates are then manually indexed, exposing the accumulated solids and precoat contained within the chambers. The operators (normally two are required) then physically remove the solids and precoat by hosing with water and/or scraping the filter cloths. Once cleaned, the press is closed, precoated and returned to the filtration mode. Total down time for cleaning and cycle preparation is approximately one hour. During the precoating, filtering and cleaning functions, the operators must take care in protecting themselves from direct exposure to the fluids to reduce the hazard of chemical burns.
In the present "Single Pass" filtrate art, covered by U.S. Pat. No. 4,428,425, a means for guarding against breakthrough of filter aid and/or solids contaminants is not provided for. Therefore it is common practice to employ a separate filter downstream to serve as a guard filter to capture the breakthrough filter aid and/or solids contaminants. Typically such a guard filter would consist of disposable cartridge filters which can be nominally or absolute rated.
Pressure leaf filters which are also used for single pass filtration are designed around a multitude of filter leaves which can be made of high density plastics and stainless steel. These leaves are available in various shapes and sizes and are covered with either a synthetic cloth or wire mesh cover. The leaf covers serve to retain the filter aid precoat material which is actually the filtering medium, not the leaf cover itself.
The filter leaves are fitted inside of a closed pressure vessel and usually spaced three inches on center, yielding a one inch cake space. The filter vessel can be verticle or horizontal.
During the filtration cycle care must be taken to prevent cake bridging between the filter leaves. Such bridging will often result in severe damage to the leaves to the extent that replacement is required.
When a preset differential pressure is reached and/or filtrate flow appreciably decreases, the unit must be taken out of service for cleaning. The first step in the cleaning operation is to evacuate the "Liquid Heel" from the vessel. The liquid heel is that volume of fluid which occupies the entire capacity of the filter tank. Depending of course on the tank size, the heel can be a sizeable volume of liquid.
The heel is removed from the vessel by introducing air or gas which causes the fluid to be displaced forward, normally to the surface volume tank. During the heel displacement, there is a high probability that much of the spent filter cake will sluff off and go forward to the surface volume tank along with the liquid phase. Since the rig's surface volume tank contains filtered fluid which has already been processed, it is not desirable to put solids back into the tank. In fact, it is counter productive to do so as those solids must ultimately be filtered again. Refiltering the contaminants and filter aid from previous filtration cycles is very costly because additional expensive rig time as well as expensive filtration services are required as a result of poor filter operation practices.
After the heel has been evacuated from the filter vessel, the leaves can be internally cleaned by "Sluicing" or by "Dry Cake Discharge" methods. The sluicing method involves spraying relatively clean liquor, in this case completion fluid already filtered, onto the solids coated filter covers until these solids are removed from cloth surfaces.
Dry cake discharge cleaning involves the mechanical scraping of the solids and precoat material after the liquid heel has been blown forward.
It should be noted that through experience the industry realizes that filter aid and contaminant solids break through may occur in pressure leaf filters much the same as in filter presses previously discussed. In fact the potential for break through is such that guard filters are usually required downstream of the pressure leaf filters. Guard filters are usually cartridge type element pods and in some cases bag filters.
Because of the problems associated with pressure leaf filters as described above, the industry has not to date utilized these types of filters as extensively as they have used filter presses for completion fluids filtration.